Electrical heating element



July 28, 1925. l,547,837

c. STEENSTRUP ELECTRICAL HEATING ELEMENT Filed March 30. 1923 Ihvehor: C hrstian Ste en strup,

Ptented July 28, 1925.

e UNITED STATES PATENT OFFICE.

CHRISTIAN STEENSTRUP, OF SCHENECTDY, NEW YORK, ASSIGNOR TO GENERAL ELECTBIC COMPANY, A CORPORATION OF NEW YORK.

ELECTBICAL EEATING ELEMENT.

Application filed Iarcl 30, 1.923. Serial No. 628327.

To all whom it may cancer n:

Be it known that 1,CHR1sTIAN STEEN- STRUP, a citizen of the United States, `residing at Schenectady, in the county of Schenectady State of New York, have invented certain new and useful Improvements in Electrical Heating Elements, of which the following is a specification The present invention relates to electrical heating elements or units'such for example, as are commonly used for household and other purposes and has for its object to improve their construction. 4

In the manufacture of these elements or units it is common practice to use what is known as sheathed wire. for the heating element. Briefly, this comprises a wire made of high resistance metal through which the heating current flows. Enclosing the wire is a steel sheath or armour and between the wire and the sheath is a closely compacted insulating material such as magnesia. After being properly formed the sheathed wire may be coiled, bent or otherwise formed into a heating .unit having the necessary elec- 'trical characteristics.

Heating units made in this matter have been incorporated in the finished article in a variety of ways, one of which is by casting. That is to say, the complete heating element is suitably placed in a mold after which molten metal, such as iron or aluminum is poured into the mold to fill the same. The metal when cooled forms a completed enclosure for ,the unit except for the terminals which project thrpigh the body for connection with the circut wires.

In the casting operation there are many rejectons due to imperfections as for eX- ample, blow holes and failure of the molten metal to completely fill the mold, all of which results in a considerable loss both in money and material. There is no fusion between the embedded article and the casting but instead merely surface contact and the heat conductivity is impaired for that reason. It-also frequently happens that even this contact is not maintained and as a result one or more small chambers or pockets are fbrmed alongside of the sheath and be tween it and the surrounding metal. Such a' pocket is a potential danger spot'because. the transfer of heat from the unit to the surrounding metal at this point is impaired `ried between rather wide limits.

with the result that the heating unit burns out or short circuits at said point. This is a Very serious matter because it is impossible to detect these pockets in the finished product by usual testing nethods and hence there s no way to ascertain that the product is defective until after it is put into use.

In carrying out my invention, the sheathed wire is formed into a heating unit of any desired shape or form. It may be used in straght lengths, coiled, bent or otherwise arranged, the only limitation being that the core wire be properly insulated from the sheath and have the necessary resistance to the flow of current and supply the necessary amount of heat for the purpose in tended. Assuming the unit to be properly formed, it is placed in a suitable container which may be made of metal or heat resisting material or a combination of both. When made of suitable metal, such as steel, the container in addition to acting as a receptacle for the heating unit also forms an integral part of the finished device. The mold is then filled with a composite metal invented by me and which forms the subject matter of another application filed of even date herewith Serial No. 628,926. Briefly stated," it comprises a basematerial comprising smooth-surfaced, self-packin particles of metal, the voids between said particles being of a capillary nature. Such particles when poured into a container freely distribute themselves or pack in much the same way that lead shot will fill a container when poured into it. By avoiding the use of pressure on the particles all danger of distorting the heating unit or parts thereof is avoided. For the purpose of the present invention it is preferable to use' small shot made of mild steel having a low carbon content. The diameter of the shot can be va- The main considerations are that the material be selfpacking and that the voids between the particles or shot be of a capillary nature; To the base metal is added a void-filling and 'fusing metal of lower melting point and in suflicient amount to fill the voids thereof. This metal is preferably supplied in the form of small pieces distributed over the top of the shot. Wherethe shot are made of steel, copper is used as the void-filling and fusing metal. To obtain the best results the inner walls of the container if it be made.

boronized copper should be used or copper that has been treated in a hydrogen furnace, to remove the impurities. At this stage in the process all the parts are or may be at room temperature.

The self-packing feature of the material and the character of the voids is of the utmost importance for this class of work.- If the partclesare rough they will not be self packing and the size of the voids will vary unduly. While-the shot may be very small, they are nevertheless of appreciable size, and

,as :hearing on this point, if the particles are reduced to powder form they will not be satisfactory because the fusin metal will not penetrate the mass and-`in owing will pick up the powder which changes its melting point. I have also found that the resulting product shrinks to a substantial degree and also Cracks and is not dense and homo geneous.

Having assem bled the heating unit within the container and filled the latter with steel shot, the next step is to place the container and its contents in a closed furnace in which an atmosphere of selected gas is constantly maintained. When steel and copper are used to form the composite enclosing metal for the unit, hydrogen gas'is best ad-apted for the purpose. The contents of the container are then heated to a temperature sufliciently high to melt the copper, which temperature will not injure the heating unit nor melt the shot. To obtain the best results the copper should be distributed rather uniformly'over the shot. As already pointed out this'may be done by distributing or sprinkling small pieces of copper over the exposed 'uppersurface of the body of shot or it may be supplied in fluid form from acoarse mesh screen or `equivalent device suitably placed 'above the container and from which the molten copper is free to flow onto the shot. My experience has demonstrated that if suflicient molten copper is permitted to accumulate at any point in the container, so as to form' a pool, it causes a disturbance of the steel shot with theresult of impairing the homogeneity of the finished product. When the necessary tem`- perature is reached the copper will flow by capillary action into' the voids between the shot v and fill them. This heating action causes fusion of the parts and also a cer tain alloyage of the copper with the sheath of the wire, with the steel shot and also with turns or parts of the heating unit and the container and the voids are filled withcopper. Due 'to this fact and to the complete fusion of the parts a very intimate therm-al connection is obtained between the wall or sheath of the heating unit and the surrounding metal and the impaired heat transmissionand pockets which are so highly objectionable in the casting process above referred to are entirely avoided.

The composite material has about the same strength as that of mild steel and can be treated in the same way. That-is to say, it can be rolled, drawn, swaged, forged and machined. This means that the surfaces 'of the completed product can be finished with' ordinary machine tools.

In the accompanyng drawing, which is illustrative of my invention, Fig. 1 is a plan view of a sheathed wire heating unit located in a container, Fig. 2 is a sectional View of the same; Fig. 3 shows a container made of heat-resisting material Fig. 4: is a plan View of a container and heating unit suitable for a fiat iron Fig. 5 shows another way for making a container; Fig. 6 shows on a greatly exaggerated scale the steel shot used as the base of the composite material, and F ig. 7 is a View of a piece of resistance material, such as wire, covered with insulation capable of withstanding high temperatures, the sh'eath being omitted.

10 indicates a container which can placed in the bottom 'of the container orit may be spaced therefrom by first pouring in some of the steel shot and leveling them ofl' to the desired thickness asin Fig. 3 for example. After the unit is placed in position steel shot'are poured into the .container to the level of the top. The container may be jarred slightly to facilitate the settling of the shot. The ends 12 of the heating unit may be bent upwardly so as to aford means for connecting the unit to the ciruit wires of the source of supply. 13 indicates the resistance wire and 14 the steel sheath therefore, there being insulation between the two. Copper in small. pieces is then distributed over the steel shot and then the container and its contents are treated in a closed furnace in a hydrogen atmosphre as above described.

Fig. 3 shows a different form of container. In this case it is composed of a body of heat-resisting material 16 which is composed of hard-pressed asbestos or asbestos compound. In .this case a layer of steel shot is placed in the container before the unit is inserted so as to provide a surface which later on can be machined. The container is tainer is then filled to the level of the top with steel shot over which the nece-ssar amount of copper is distributed, after which the con laced in aclosed furnace as before. This type of container may be made in one or more pieces.

Fig. 4 shows my invention applied to a flat-iron. In this case a shallow sheet metal container 17 is pressed 'ip to the desired shape and size, after which thesheathed wire heating unit 18 is inserted. It may rest directly on the bottom of the container or be spaced thereirom in the manner previously described, Subsequent Operations are the same as those above recited. The shoe thus formed may be used with any desired form of holder.

In Fig. 5 is shown a container composed partly of metal in the form of Vertical walls 19 and a base of heat-resisting material 20.

Fig. 6 shows on a greatly exaggeratedv scale the steel shot 21 employed in the process, said shot being as above stated of such size in actual practice that the voids between them are of a capillary nature.

In addition to the advantages specifically set forth above, the resulting product may be made very thin, and in any case the enclosing metal will be dense `and homogeneous throughout. In the making of pots, kettles, etc., theheating unit may be located directly in the bottom and fused to it and the side walls by the methods .herein described. I

Where a pressed metal container is em ployed for the heating unit it is unnecessary for most purposes to finish the outer surface thereof unless a special finish is desired, as for example .one of nickel plate. The finish resulting from the type of molds shown in Figs. 3 and 5 is good enough for many purposes, and where it is' not, a sim-' ple machine operation will suflice.

In another, and in some respects, a simpler form of my invention the steel sheath is omitted altogether and the resistance ma terial 13 F ig. 7 is coated with' an insulating material 22 which .of itself is capable 'of withstand ing the necessary high temperature to melt the copper without disintegra- ,ti`on or injury. In such a case the resistance material 'may 'be coiled, bent or otherwise formed to sha-pe and afterwards `coated with insulation, or the insulation may be applied before the forming operation whichever is the most convenient. After the heating unit is thus ormed it is placed in a, container and the surrounding portons of the chamber filled with steel shot as before, so as to embed the unit therein. The subsequent Operations are the' same as those already described. The transfer of heat from the unit to-the holder or supporting material is about the same in both cases because substaitially the same amount 'of sulation has to be used in any event to elecing unit composed of resistance material, v

insulation therefor, and a holder in which the unit is embedded, comprising a base of self-packing metal particles and a metal having a lower melting point than the said particles which fills the voids and unites them intoa unitary mass, which mass has greater strength than said metal of lower melting point.

2. The combination of an electrical heating unit composed of resistance material, an insulating envelope therefor, and a holder in which the unit is embedded comprising a base of steel shot and another metal having a lower melting point than the shot which fills the voids between the shot and unites them, said holder having a strength substantially the same as the shot.

3. The combination of an electrical heating unit composed of resistance material, an insulating envelope therefor, a container in which the unit is located, and 'a filler for the container in which the unit is embedded comprising a base of steel shot and copper said copper' uni ting the shot and the wall of the container and filling the voids between them( 4. The combination of a resistance material which is enclosed in a metallic sheath to form a heating element and from which it is insulated with a holder therefor of composite metal comprisng a base of self-packing metal particles and another metal having a lower melting point than 'the base material which fills the voids in the base material and unites said particles with the sheath. 5. The combination of resistance material form a heating element and from which it is' electrically insulated with a holder in voids between them of a capillary nature,.

and another metal having a lower melting point than the sheath and the particles said other metal' filling the voids and intimately uniting the said particles to the sheath.

6. The combination ofresistance ,material which is enclosed in a' metallic sheath to form a heating unit and from which it is in*- sulatedlwith adiolder therefor of composite ,metal comprising smallmetallic shot and a. *metal which has a lower melting point than 'the shot, fills'the voids between them and the which is enclosed in a metallic sheath to sheath and unites the shot and-sheath into a unitary structure, said composite metal having greater strength than the metal of lower meltng point.

7. The combination of a heatiaig element which is. enclosed in a metallic sheath with a holder of composite material in which the element is embedded comprising a base material composedof small steel shot and copper which fills the Voids between the parts and unites them, said composite material being stronger than the copper.

8. The combination of an insulated heating element, a metallic container in which the elenent is located, and a filling for the container and in which the element is embedded comprising a base material of small self-packing metal particles and another metal of lower melting point which fills the voids in the base material and unites said particles with the container.

9. An electrical heating element which is enclosed in a metallic sheath, a metallic container in which the element is located in spaced relation to the walls thereof, a filling for the container comprising a base of small self-packin, metallic particles having vods of a capilla'y nature, and another metal having a lower melting point which fills the VOldS between said particles and unites them with the sheath and the walls of the container.

10. An electrical heating element which is enclosed in a metallic sheath, a shallow steel container in which the element is located in spaced relation to the walls thereof, a filling for the container comprising a base of steel -shot having voids of a capillary nature, and

copper which fills the voids between the shot and unites them with the sheath and the walls of the contain'er.

11. As an article of manufacture, a steelsheathed electrical heating element which is embedded in composite metal comprising small steel balls and Copper which copper unites the steel .balls with the steel sheath, said composite metal having greater strength than copper.

12. An electrical heating element which is enclosed in a metallic sheath andinsulated therefrom, a composite metal holder for the element which comprises self-packing metal particles which are in contact with each other and with the sheath and a metal which has a lower melting point than the sheath and particle's, fills the Voids between them and is slightly alloyed with the surface' portions of said parts.

s I l :is

In witness whereof, I have hereunto set 'my hand this 29th day of March, 1923.

CHRISTIAN STEENSTRUP. 

